LONDON, England --Harold Shipman was the silent suburban killer, targeting elderly women with names like Norah, Winifred, and Irene, and killing them behind the lace curtains of their own homes.

The number of victims might never be known. It could be more than 200.

The motives of Britain's worst mass killer remain a mystery. Prosecutors said he had a lust for power over life and death and a desire to "play God." He also tried to gain financially from his victims.

But the father of four, who denied murdering 15 women at his trial which ended in January 2000 with conviction and a sentence of life in prison, never gave reasons.

Failures by police investigators allowed Shipman to continue his killing spree undetected, an inquiry ruled on July 14, 2003.

An earlier inquiry said he murdered 215 of his patients between 1975 and 1998.

Almost from the beginning of his 24-year career as a general practitioner in Hyde on the outskirts of Manchester, England, Shipman is believed to have started administering deadly doses.

He would inject his patients, mainly women aged between 75 and 84, with diomorphine -- the medical term for heroin -- during his afternoon rounds.

Shipman was well versed in the administration of drugs having been an addict during the 1970s, leading to his conviction in 1976 for forging prescriptions for his own use.

More than half of the 521 death certificates he wrote were likely to have been premature, according to a British government audit report.

And he often kept husbands and siblings away from the patients' bedsides in their dying moments when he was around.

Softly spoken, greying, bearded and bespectacled, Shipman would comfort dying victims. Later, he would enter "heart problems," "stroke" or "old age" on the death certificate.

He forged the will of his last victim, 81-year-old Kathleen Grundy, so he could inherit £386,000 ($610,000), but it was this final crime that tripped Shipman.

The alarm was raised after Grundy's daughter, a lawyer, became suspicious about her mother's will bestowing her entire estate to Shipman.

Shipman began his killing in 1975, a year after he began practicing as a GP in Yorkshire. But the bulk of his victims died after he had moved to Hyde.

When passing sentence on Shipman at his trial in January 2000, Judge Thayne Forbes, said: "None of your victims realized yours was no healing touch."